In a small bowl, mash the avocado with lemon juice, salt, and pepper until smooth.
In another bowl, mix the cream cheese with chopped dill until well combined.
Toast the whole-grain bread slices to your liking.
Spread the dill-infused cream cheese on two slices of the toasted bread.
On the other two slices, spread the mashed avocado mixture.
Layer the smoked salmon evenly over the cream cheese spread.
Top the salmon with thinly sliced red onions and a handful of arugula.
Assemble the sandwiches by placing the avocado-spread slices over the salmon layers.
Gently press the sandwiches together and slice them in half if desired.